With respect to separation of mixtures, there are various techniques for removing substances (e.g., salts) dissolved in a solvent (e.g., water). In recent years, however, membrane separation methods are coming to be utilized increasingly as processes for energy saving and resource saving. Examples of the membranes for use in the membrane separation methods include microfiltration membranes, ultrafiltration membranes, nanofiltration membranes, and reverse osmosis membranes. These membranes are used in the case of obtaining potable water, for example, from seawater, brackish water, or water containing a harmful substance, and for producing industrial ultrapure water, wastewater treatments, recovery of valuables, etc.
Most of the reverse osmosis membranes and nanofiltration membranes that are commercially available at present are composite semipermeable membranes, and there are two kinds: ones including a microporous supporting membrane and, disposed thereover, a gel layer and an active layer formed by crosslinking a polymer; and ones including an active layer formed by condensation-polymerizing monomers on a microporous supporting membrane. Among such composite semipermeable membranes, composite semipermeable membranes (Patent Documents 1 to 4) obtained by coating a surface of a microporous supporting membrane with a separation functional layer including a crosslinked polyamide obtained by the polycondensation reaction of a polyfunctional amine with a polyfunctional acid halide are in extensive use as separation membranes having high permeability and selectively separating properties.
However, there is a problem in that in cases when composite semipermeable membranes are continuously used, the separation performance of the membranes decreases due to contact with oxidizing substances, such as free chlorine, which are contained in water to be treated. In addition, fouling substances adhere to the membrane surfaces with the lapse of use, resulting in a decrease in the permeation flux of the membranes. Although it is therefore necessary to conduct cleaning with a chemical such as an acid after performing the operation for a certain period, there is a problem in that this cleaning reduces the separation performance of the membranes.
Consequently, it is desired to develop a composite semipermeable membrane which has high oxidation resistance and changes little in membrane performance through cleaning with a chemical such as an acid, that is, has high acid resistance, in order to continue a stable operation over a long period.
In Patent Document 5 is disclosed a method in which a persulfuric acid salt is brought into contact with a composite semipermeable membrane, as a means for improving oxidation resistance. In Patent Document 6 is disclosed a method in which a cyclic ester of sulfuric acid is brought into contact with a composite semipermeable membrane, as a means for improving acid resistance.